1. Field of the Invention
The present invention relates to a method for data communication between a base station and at least one transponder by a high-frequency electromagnetic carrier signal, onto which information packets are modulated, whereby each information packet has a header section, a middle section, and an end section and whereby the middle section has a data field for the transmission of data and/or address information and a protection field placed thereafter for the correction of errors in the transmission of the data and/or address information. The invention relates further to a base station for carrying out this method and to a data communication system containing a base station and at least one transponder in communicative wireless contact with the base station.
2. Description of the Background Art
The invention falls within the realm of transponder technology and in particular within the field of contactless communication for the purpose of identification. Although it can be used in principle in any communication system, the present invention and its underlying problem are explained below with reference to RFID communication systems and their applications. Here RFID stands for “radio frequency identification.” Reference is made on the general background of this RFID technology to the “RFID-Handbuch” (RFID Handbook) of Klaus Finkenzeller, Hanser Verlag, third updated edition, 2002.
In the case of transponders, an electromagnetic signal transmitted by the base station is received by the transponder and demodulated. Active, semipassive, and passive transponders are differentiated here depending on the design of their energy supply. In contrast to active transponders, passive transponders do not have their own energy supply, so that the energy necessary in the transponder for demodulating and decoding the received electromagnetic signal must be obtained from this electromagnetic signal itself transmitted by the base station. In addition to this unidirectional energy transfer, bidirectional data communication typically occurs also between the base station and transponder.
Bidirectional data communication between the base station and transponder typically includes an interrogation sequence and a response sequence.
The basis for the bidirectional data transmission between the base station and transponder thereby forms a so-called communication protocol, which specifies, in addition to the data information to be transmitted, also the control information for the data communication.
A prior-art RFID communication protocol for a known data communication between a base station and transponder is described in the Unexamined German Patent Application DE 101 38 217 A1, which corresponds to U.S. Publication No. 2003133435. Accordingly, an information packet to be transmitted from the base station to a transponder has at least one header section, a middle section, and an end section. The header section defines the number of data to be transmitted and their identification. The middle section contains the data to be transmitted in each case. The end of the information packet is communicated in the end section to the receiver of the data transmitted in each case. The data communication is protected with protection mechanisms, such as, for example, a CRC protection field or parity bits.
A generic RFID method and system for bidirectional data communication is also the subject of the so-called Palomar Project, which was established by the European Commission as part of the so-called IST program. With respect to the content of this Palomar project, reference is made to the related, generally available publication of the European Commission of Jan. 11, 2002, which corresponds substantially to the ISO standard 18000-6.
For further background on bidirectional data communication between a base station and transponder, reference is made further to the Unexamined German Patent Applications DE 102 04 317 A1, DE 100 50 878 A1, DE 102 04 346 A1, which correspond to U.S. Publication Nos. 2005094720, 2002044595, and 2005128130, respectively, and which are all herein incorporated by reference, and to European patent EP 473 569 B1, which corresponds to U.S. Pat. No. 5,345,231.
In general, methods for data communication between a base station and a transponder are used to perform an identification within an authentication process, for example, with the use of RFID technology. So that, if possible, the specific communication participants notice no delay during this data communication, the data communication must typically be completed within a time-limited span of about 100 ms. This predefined time span results particularly from the relevant HF regulations and the fact that the transponders frequently move within the field of the carrier signal transmitted by the base station. Because of reflections, field cancellations constantly occur, so that the data communication between the base station and transponder is to be completed between such field cancellations to assure proper data communication. If the authentication process has not been completed within this frequently fixedly predefined time interval, then a unique identification is usually not possible, so that the particular authentication process must be repeated. This is associated with a time delay, however.
Another aspect is that, because of increasing security requirements, a plurality of data must be transmitted by a modulated carrier wave within an increasingly shorter time. For modulation, an amplitude modulation (ASK) is very frequently used in the forward and return link. In the forward link, particularly in passive RFID systems, no phase modulation (PSK) or frequency modulation (FSK) is typically used, because in this case a mixing circuit would be necessary in the transponder, for example, to recognize a phase change. This type of mixer circuit, however, uses an extraordinary amount of current, which is frequently not available particularly in passive transponders, which only have a local energy supply. In addition to the amplitude modulation, a phase modulation is also being used increasingly in the return link for data transmission use. To achieve a higher data rate and thereby a greater range, high-frequency carrier frequencies in the range of the UHF frequency or microwave are used for data communication.
The duration of a bidirectional data communication between a base station and at least one transponder, which represents an essential parameter in RFID systems, depends inter alia on the following parameters:
the number of information packets necessary for a data communication;
the number of data transmitted per information packet and thereby the bit width of an information packet;
the type of data transmission (full duplex, half duplex); or
the nature of the electromagnetic field of the carrier wave and its environment (electromagnetic interference fields).
Because of the above parameters and the fact that the transponder at times moves within the electromagnetic field of the carrier wave relative to this wave, the maximum duration of a data communication, which is available for a complete transmission of data between a base station and transponder, is physically limited. In particular, the times for this data communication also depend greatly on the specific desired application.
Without the provision of additional measures, in each case a complete information packet must be transmitted at present in the forward link and subsequently also in the return link, also when a data communication in the forward link and/or in the return link is faulty, for example, by overlapping of interfering signals. This is perhaps disadvantageous primarily for information packets which are very long in time and have many symbols, because even when a data communication for an information packet was already recognized as faulty, it must be continued until the end of the particular data communication.
UHF and microwave transponders used to date employ asynchronous communication methods, in which no internal clock is transmitted with the carrier wave, for the data communication. These communication systems therefore could be operated only in half-duplex operation. Modern communication systems operate synchronously; i.e., the base station and the at least one transponder in communicative contact with this base station operate with the same internal clock. This opens new opportunities in data communication between a base station and transponder. In particular, these can now be operated using full-duplex operation after synchronization, which greatly shortens the duration of data communication. It is also necessary thereby to reduce the number of notches as much as possible, because these produce an undesirable spectrum. In particular, the notches can undesirably generate interfering signals, which can influence very weakly formed sidebands, which is a disadvantage particularly for data retransmission in the return link. These interfering signals can at times interfere with the RFID communication system to such an extent that demodulation and thereby decoding are no longer possible or can be determined only with high computational effort. At times, the received power of these received interfering signals is so great that it can cause jams in the RFD communication system. To prevent this, the data communication may be activated only in such timeslots, during which the other interfering transmitting systems transmit no or only minor interfering signals. In a very high-interference environment, these timeslots must be limited in time in such a way that for a high efficiency the RFID communication system must be capable of supporting the shortest communication times possible.
However, this does not apply to existing RFID communication systems. This results from the fact that the structure of the corresponding RFID communication protocols, including their security level, are typically fixedly predefined. In particular, the bit width and thereby the duration of an information packet is fixedly predefined within an application and therefore can no longer be changed. In the case of a highly interfering environment, thereby data communication between a base station and transponder particularly for very long information packets and thereby for a very large volume of data to be transmitted per information packet is no longer possible or possible only with the acceptance of a more or less highly noisy data communication, because it is almost impossible to transmit all symbols trouble-free for the duration of an information packet.
Another physical problem in data communication during use of UHF or microwave signals is the presence of so-called “nulls”. These “nulls” arise by constructive and destructive overlapping of the carrier signals, transmitted by the base station, by reflection off objects in the vicinity of the base station. This can result in the cancellation of parts of the transmitted signal, which ultimately also precludes trouble-free data transmission. Because of this interference phenomenon, however, an RFID data communication is no longer possible or possible only with limitation, because energy is no longer available to the transponder in the indicated cases. The transponder, however, requires this energy for data retransmission within the scope of a backscatter process. Here as well, RFID communication systems and methods for RFID data communication are required which are capable of keeping the time necessary for data communication between a base station and transponder as short as possible.
Another problem in existing RFID communication systems, which are operated within the UHF or microwave range, is that the speed of data transmission is sometimes drastically limited by national or supraregional HF provisions. In particular, national regulations within the territorial application of the Federal Republic of Germany, as well as regional provisions within the territorial application of the European Union, provide that the maximum transmitting power of an electromagnetic carrier wave during operation of an RFID system and also the corresponding carrier frequency are to be limited to such an extent that, on the one hand, other radio systems are not affected thereby and, on the other, health-related limits are not affected. Moreover, RFID systems in Europe are operated with a duty cycle that corresponds maximally to about 10% of the time during which the frequency band may be occupied. Subsequently, the base station must wait 90% of the time, which results in data communication protocols having to be as short as possible to ensure an effective, highest possible data rate. Therefore, here as well, there is the need for shorter times for data communication between a base station and transponder.